The issue was milk price supports; the setting was the Senate Agriculture subcommittee on commodities; the lesson was that President Reagan's budget-cutting crusade won't be easy.

The two senators from Wisconsin taught the day's lesson. Both of them are ardent budget-cutters. William Proxmire, a Democrat, prides himself on a reputation as an antispender. Robert W. Kasten Jr., a conservative Republican, won in November on Reagan's coattails, which he had grasped avidly.

Yesterday Kasten opened his testimony by saying: "It would be easy for me to sit here this morning and play 'politics as usual' . . . [and] plead with you to let my Wisconsin dairy farmers alone. . . . All of us in Congress can quite cheerfully support budget cuts as long as they don't effect our constituents. . . . But the economic facts are simple. Everyone -- and that means your constituents as well as mine -- has to assume a fair share of the budget cuts that are so essential. . . ."

That was the introduction; but then Kasten got to the administration's proposal to skip a scheduled April 1 increase in the dairy support price on the grounds that huge existing surpluses mean the support price is too high. Suddenly his testimony veered in a new direction:

"I simply cannot and will not support any cutbacks in the dairy program that are not keyed to a total package of reductions. . . . Wisconsin's dairy farmers -- and this senator -- are not willing to take the brunt of cuts in the agricultural sector unless other segments of that community are going to absorb comparable cuts. . . ."

Proxmire did not mention politics as usual. He did say he was "one who fully supports the president's proposed reductions in our federal budget. I think we can and should go further than the president has requested in those reductions. The president deserves great credit for proposing these deep and painful reductions. He is right. But on dairy price supports he is wrong."

Proxmire then gave an eloquent defense of the dairy program, noting that real farm income has been falling; that diversified farmers have been concentrating on dairy production not so much because other areas of farming are so depressed; and that, according to some studies, dairy farmers (who must milk their cows twice a day, 365 days a year) actually earn less than the minimum wage. No Golden Fleece awards here. If anything, this program should be more generous than it is, Proxmire suggested.

This was a strange day in the Senate Agriculture Committee, as Sen. Roger W. Jepsen (R-Iowa) pointed out at one juncture when he found himself on the same side of the argument as Ellen Haas of the consumer-oriented Community Nutrition Institute, an activist outfit for which Jepsen could not hide his contempt.

Haas testified in favor of the administration proposal to skip the April 1 increase in price supports, which under existing law is required to compensate farmers for inflation in other areas of the economy.

Jepsen didn't fancy the support of consumer activists, but he had decided to support the administration too. The current level of milk price supports is "adequate," Jepsen said.

Representatives of two giant cooperatives representing dairy farmers -- cooperatives that gave hundreds of thousands of dollars to members of both houses in last year's election campaign -- were on hand to disagree forcefully with Jepsen and anyone else who thought they didn't deserve their increased price support.

Most revealing, however, was the dairy lobbyist who wasn't there. Patrick B. Healy of the National Milk Producers Federation, the leading Washington spokesman for dairy farms, was nowhere to be seen at yesterdayhs hearing, and his federation had absolutely nothing to say about this matter of apparently vital concern to dairy farmers.

Healy was absent because he is busy trying to make a deal with the Reagan administration. His sensitive political antennae convinced Healy months ago that the skyrocketing costs of the dairy program -- $1.3 billion in fiscal 1981, an estimated $2 billion or more in 1982 -- spelled political trouble.

He has taken the position that the dairy industry should volunteer to give up the April 1 price support increase in return for other concessions that would protect dairy farmers' incomes. Healy favors letting the basic price support level fall from 80 to 75 percent of "parity," the traditional measurement of relative farm income, if a deal can be struck.

The board of the Milk Producers Federation has agreed with Healy, though some of the co-ops that belong to the group disagree. Such a split in the ranks of milk producers is rare.

But Healy has hardly gone into the tank for the Reagan administration. He wants to retain healthy milk price supports, healthier than Budget Director David A. Stockman has made room for in his budget projections. Healy has told the Department of Agriculture that if a satisfactory new deal isn't worked out soon, he will join in efforts to block the administration's attempt to skip the April 1 price support increase when the idea comes up in the House.

Deputy Agriculture Secretry Richard Lyng urged Senate passage of the administration proposal yesterday, noting that April 1 is fast approaching, and that $147 million can be saved this year if the bill is passed.